A conventional power supply for an LED lamp takes power from an input line at one voltage (typically 12V AC 50/60 Hz) and converts it to a higher DC voltage (e.g., 30 V DC) to power the LEDs. The temporal characteristics of the power signal directly impact the quality of the light generated by the LED. Thus, the power supply also regulates the current to the LEDs to provide consistent lighting output.
Due due to the zero crossings of the AC signal, which occur at twice the AC frequency, the power supplied to the LED is momentarily at zero. This leads to what is referred to as systematic flicker, which although may not be directly observable, nonetheless leads to perceptible degradation in the quality of the light generated by the LED. During these very low voltage points of the AC input or when the AC input is interrupted by a phase-cut dimmer, it is desirable to continue to provide power to the LEDs to prevent stroboscopic flicker.
In addition, noise and other disturbances in the electric power signal also degrade the performance of sourced LEDs. Thus, it is desirable to mitigate any noise or other power line disturbances in the power signal.
In order to alleviate both systematic flicker, power line disturbances and noise, an energy storage device such as a capacitor may be introduced between the power source and the LED. The energy storage device acts as a buffer and is designed to have enough capacity to continue to power the LED while the AC signal crosses zero. In general, the higher the voltage established on the energy storage device, the more immune the power supply is to systematic flicker and power line disturbances. Preferably, this solution utilizes a two-stage approach comprising a first stage introduced before the energy storage device and a second stage introduced after the energy storage device.
The first stage may be a voltage converter, which functions to fill the energy storage device. This converter allows for optimized input power draw from the line (high power factor (“P.F.”) for example). Because boost converters have significantly better P.F. than buck converters, they are used almost exclusively as the first power conversion stage in a two-stage arrangement. The intermediate DC voltage on the storage capacitor (output of the first stage) must be approximately twice the input RMS voltage for the boost converter to have high P.F.
The second stage may also be a voltage converter, which functions to draw energy from the energy storage device to drive an LED. The second stage allows for a highly uniform low or zero-ripple output to the LEDs. The second stage is typically a buck stage, which functions to reduce the voltage level at the storage capacitor down to the level of the LED with output current regulation as the main operating mode.
In this arrangement, the higher the intermediate voltage, the smaller the required storage capacitance to hold the LEDs up through the dropout periods. However, as this voltage is increased, each converter becomes less efficient. In very small lamps such as the MR16, this leads to a very challenging tradeoff between efficiency, cost, and lamp size. Typical efficiencies for boost and buck converters with 3:1 transformation ratios might be ˜87%. The net efficiency of this combination is thus ˜75%, a significant reduction.
With a buck stage, the input voltage must be higher than the output. Generally speaking, in the prior art the nominal voltage at which this capacitor operates is a fixed parameter such as 45 Volts. In some conventional power supplies, the intermediate capacitor voltage can vary but usually does so as a function of the type of power grid to which it is connected. For example, some power supplies allow the intermediate capacitor voltage to be 240 VDC when the input voltage is 120 VAC, and allow the capacitor voltage to rise to 380 VDC when the input voltage is 230 VAC. Most prior art two-stage power supplies fix the capacitor voltage (in this example) to the higher of the two (380 VDC) to allow the device to operate from either input voltage. (It is not permissible in this example for the input voltage to be 230 VAC while the output voltage is 240 VDC.)
FIG. 1 shows a conventional two-stage driver. Input power source 110 provides alternating voltage (“AC”) signal AC (not shown in FIG. 1). Two-stage driver 100 comprises boost stage 104 and buck stage 106. AC/DC converter 130 converts AC signal generated from input power source 110 to a DC signal (not shown in FIG. 1), which is provided to boost stage 104. Boost stage 104 may further comprise inductor 134(1), diode 132(2) and switch 136(1). Boost stage 104 performs voltage conversion of the DC signal generated by AC/DC converter 130 to generate an output voltage signal (not shown in FIG. 1). The output voltage signal from boost stage 104 is provided to capacitor 112, which stores energy in electromagnetic form.
Buck stage 106 draws energy from capacitor to power LED 108. Buck stage 106 may further comprise inductor 134(2), diode 132(2) and switch 136(2).
The input power of boost stage 104 is controlled by capacitor voltage control system 102 so that under typical operating conditions, the capacitor voltage (average, peak or some other measure) is held constant. The lowest undulation of the capacitor voltage must always be higher than the forward voltage of LED 108 in order to maintain the flicker-free output condition.
Eventually capacitor 112 ages and its capacitance is insufficient to prevent output ripple or possibly severe flicker. Also, there is typically a design margin required on the set-point of the capacitor voltage (perhaps 25% higher than the LED voltage), which can significantly reduce the efficiency.
Applicant has identified significant shortcomings in the conventional driver 100 as depicted in FIG. 1. First, although the cascaded efficiency reduction of two power converters may be tolerable in applications in which the power supply is not inside a LED or lamp, inside an LED or lamp, the thermal conditions usually limit or define the performance envelope of the lamp. Furthermore, the lifetime of the electrolytic capacitor 112 decreases exponentially with operating temperature. For example, a power supply with a capacitor, which operates at a temperature of 40 C may last in principle for 150 continuous years of service or more before its electrolytic capacitor wears out. That same capacitor in a lamp operating above 100 C may last only 1/60th as long, only a few short years. In a typical two-stage power supply, when the capacitor's value drops below a certain design level (due to this aging process) it will no longer meet its specifications or may malfunction in an unpredictable way. The present invention addresses many of these shortcomings and fulfills one or more of these needs among others.